These appliances have one or more sensitive microphones that pick up not only the user's voice but also the surrounding noise, which noise constitutes a disturbing element that, under certain circumstances, may go so far as to make the speaker's speech unintelligible. The same applies if it is desired to implement voice recognition techniques, since it is very difficult to perform shape recognition on words buried in a high level of noise.
This difficulty associated with surrounding noise is particularly constraining for “hands-free” devices in motor vehicles, regardless of whether the devices comprise equipment incorporated in the vehicle or accessories in the form of a removable unit incorporating all of the components and functions for processing the signal for telephone communication.
The large distance between the microphone (placed on the dashboard or in a top corner of the ceiling of the cabin) and the speaker (whose position is determined by the driving position) means that a relatively high level of noise is picked up, thereby making it difficult to extract the useful signal that is buried in the noise. Furthermore, the very noisy surroundings typical of the car environment present spectral characteristics that are not steady, i.e. that vary in unpredictable manner as a function of driving conditions: passing over a bumpy road or cobblestones, car radio in operation, etc.
Difficulties of the same kind occur when the device is an audio headset of the combined microphone and earphone type used for communication functions such as “hands-free” telephony functions, in addition to listening to an audio source (e.g. music) coming from an appliance to which the headset is connected.
Under such circumstances, it is important to ensure sufficient intelligibility of the signal as picked up by the microphone, i.e. the speech signal from the near speaker (the wearer of the headset). Unfortunately, the headset may be used in an environment that is noisy (metro, busy street, train, etc.), such that the microphone picks up not only the speech of the wearer of the headset, but also surrounding interfering noise. The wearer is indeed protected from the noise by the headset, particularly if it is a model having closed earpieces that isolate the ears from the outside, and even more so if the headset is provided with “active noise control”. In contrast, the remote speaker (the speaker at the other end of the communication channel) will suffer from the interfering noise picked up by the microphone and that becomes superposed on and interferes with the speech signal from the near speaker (the wearer of the headset). In particular, certain speech formants that are essential for understanding voice are often buried in noise components that are commonly encountered in everyday environments.
The invention relates more particularly to de-noising techniques that implement a plurality of microphones, generally two microphones, in order to combine the signals picked up simultaneously by both microphones in an appropriate manner for isolating the useful speech components from the interfering noise components.
A conventional technique consists in placing and pointing one of the microphones so that it picks up mainly the speaker's voice, while the other microphone is arranged so as to pick up a noise component that is greater than that which is picked up by the main microphone. Comparing the signals as picked up then enables the voice to be extracted from the surrounding noise by analyzing the spatial consistency between the two signals, using software means that are relatively simple.
US 2008/0280653 A1 describes one such configuration, in which one of the microphones (the microphone that mainly picks up the voice) is the microphone of a wireless earpiece worn by the driver of the vehicle, while the other microphone (the microphone that picks up mainly noise) is the microphone of the telephone appliance, that is placed remotely in the vehicle cabin, e.g. attached to the dashboard.
Nevertheless, that technique presents the drawback of requiring two microphones that are spaced apart from each other, with its effectiveness increasing with increasing distance between the microphones. As a result, that technique is not applicable to a device in which the two microphones are close together, e.g. two microphones incorporated in the front of a car radio of a motor vehicle, or two microphones arranged on one of the shells of an earpiece of an audio headset.
Another technique, known as “beamforming”, consists in using software means to create directivity that serves to improve the signal-to-noise ratio of the microphone array or “antenna”. US 2007/0165879 A1 describes one such technique, applied to a pair of non-directional microphones placed back to back. Adaptive filtering of the signals they pick up enables an output signal to be derived in which the voice component is reinforced.
Nevertheless, it is found that such a method provides good results only on condition of having an array of at least eight microphones, with performance being extremely limited when only two microphones are used.